(a) Field of the Invention
The present invention relates to a cathode ray tube (CRT) and, more particularly, to a CRT which can minimize raster distortion of electron beams while maintaining structural strength of a shadow mask.
(b) Description of the Related Art
Generally, a faceplate panel for CRTs has a convex lens shape with inner and outer curved surfaces. The convex lens-shaped panel has advantageous in various aspects such as convenience in formation, stability in strength and adaptability for the shadow mask application.
However, to the eye of the viewer, it is desirable that the screen image should be displayed to be substantially flat. For this purpose, several attempts have been made to form the inner and outer surfaces of the faceplate panel with a flat shape while maintaining normal display characteristics of the CRT. It is found that when the flat panel is employed for the display screen use, there occur problems in convergence characteristic of electron beams and strength of a shadow mask. For instance, since the flat-shaped inner surface of the panel is naturally formed with a flat phosphor screen, it becomes difficult to deflect three electron beams of red R, green G and blue B on correct phosphors on the phosphor screen. Furthermore, since the shadow mask facing the flat-shaped inner surface of the panel should be also flat, the desired strength of the shadow mask cannot be achieved through the common shadow mask forming technique.
In addition, there is a problem with the flat-panel CRT that is derived from the standpoint of the viewer. When the viewer watches a monitor with the flat-shaped panel, he feels that the screen image is sunken at its center portion while protruded at its peripheral portion.
Therefore, it is preferable that the outer surface of the panel is flat whereas the inner surface of the panel is curved.
In such a faceplate panel, as the overall curvature radius of the inner curved surface of the panel becomes smaller, the panel is more easily produced and the shadow mask has a more stable structure capable of reducing a doming phenomenon. However, when the curvature radius falls short of a minimum effective value, the peripheral portion of the panel bears an undesirably large thickness and this results in poor production efficiency as well as high production cost. Furthermore, the transmission rate of the peripheral portion becomes poor due to its large thickness, causing brightness failure.
In order to overcome such problems, various techniques are proposed for the one-sided flat panel CRT application. For example, these techniques are disclosed in Japanese Patent Laid Open Publication Nos. 6-36710 and 6-44926. However, they do not specify technical details for preserving the structural strength of the shadow mask which should be re-designed pursuant to the curvature radii varying at different positions of the inner curved surface of the panel. Furthermore, they do not discriminate the desired thickness ratios of a diagonal portion of the panel to the peripheral portion for minimizing distortion of the screen image. Therefore, when the CRT panel is manufactured on the basis of the above-identified techniques, the aforementioned problems remain to be unsolved.
In the usual sized flat-panel CRTs of 21-inch, 25-inch and 29-inch, it turns out that the thickness ratios of the peripheral portion of the panel to the center portion are 3.13, 2.91 and 2.72, respectively. These ratios are so high that they result in poor production efficiency as well as brightness failure.